This invention relates to a heat exchange device and, more particularly, to an air cooled vacuum producing condenser. Specifically, the invention relates to an air cooled heat exchanger wherein an air moving means induces air flow over heat exchange means and outwardly to atmosphere through a plenum or exhaust stack.
An air cooled heat exchanger generally consists of a tubular heat transfer surface and a fan. Fluid passing through the tubes surrenders heat to air propelled by the fan over the external tube surface. The heat energy of air passing over the tubes increases in the same amount as the heat energy lost by the fluid, assuming that no other heat losses occur. A reduction of the air mass passing over the heat transfer surface will reduce the amount of absorbed heat by a proportionate amount. Therefore, the cooling capacity can be controlled by varying the air mass passing over the tubular heat transfer surface.
This invention is particularly adapted for relatively large capacities as may be required for condensing the exhaust of steam turbines. In large capacity installations, a plurality of fans may be utilized, the fans having a plenum in common on their suction side of each having its own individual exhaust stack for discharging the air from the interior chamber of the condenser. One method of reducing the air mass passing over the heat transfer surface of such large capacity units is to simply shut down one or more of the fans. This measure will conserve fan energy in colder weather when operation with fewer fans will provide the necessary cooling capacity. An individual fan also may be rendered inoperative due to malfunctions. The fan may have to be removed for repair or replacement. If a fan is shut down for one reason or another, a back draft problem is created. In other words, the remaining operative fans will induce a downward flow of air through the ring or cylinder of the inoperative fan and thereby reduce the flow of air over the heat transfer surface.
Heretofore, continued operation of such a unit with one of the fans inoperative required covering the respective fan with a blocking means, such as a common board, to block any downward air flow through the fan induced by the remaining operative fans. This has been a rather crude expedient. Covering the fan ring or cylinder of an inoperative fan would be too laborious or even impractical if individual exhaust stacks are used. Covering the top of the stack and preventing the cover from falling off can also be considered a cumbersome undertaking. Back draft dampers placed permanently in the vertical air exhaust channel to open by the velocity pressure of the moving air would debilitate the fan performance.
This problem is further compounded in condenser units where some or all of the exhaust air in the plenum is recirculated through a port in the exhaust stack and back over the heat transfer surface for adjusting the cooling capacity of the unit. If a particular fan is rendered inoperative and the fan ring or cylinder is too inaccessible for being covered, the other alternative of covering the top of the stack would allow the remaining operative fans to cause a back draft of air through the recirculating port of the stack and thence through the ring of the inoperative fan into the plenum, thereby reducing the flow of air over the heat transfer surface.
There is a definite need for a new and improved heat exchange device which has means for selectively blocking the exhaust stack to prevent back draft through the condenser when a particular fan is inoperative. This invention is directed to solving such problems.